Engine Knock Detection – Are we over-complicating things?
June 19, 2026I came across this engine knock detection product, the Knock Detective from FWLR. Product looks great, it’s compact (contained in a 52mm gauge housing), it has a 3.5mm jack to listen to knock in real time (which is a really nice touch, saves a lot of faff over a DIY setup) and knock is displayed with a series of multi-coloured leds. It can also output the knock sensor voltage signal to an aftermarket ECU.
Frequency
However one thing the unit does not have is a user programmable knock frequency detection window. The Version 2 unit does have some windowing built in but we don’t know the frequency window.
And this brings me to the question.
Plex, MoviChip and Tuner Nerd (and others) knock detection products have a user adjustable frequency detection window but Knock Detective does not.
So are companies over complicating the knock detection process?
Do we really need to know the frequency of the engine knock to detect the knock?
Pros – Knock Detection without Frequency Window
Generally, in my experience, when an engine knocks the sound can be higher than all other noises. In other words, if we are just looking for the loudest sound (highest voltage output from the knock sensor) we can detect knock.
And if an engine is experiencing reasonable/bad knock it can distort the signal from the knock sensor. Instead of hearing sound/noise from the engine, the signal starts getting clipped (like a speaker distorting). In this situation, I think a case can be made that knowing the frequency of the knock is not necessary as the signal from the knock sensor will be maxxed out/clipped/distorted.
In short, knock can be detected without a frequency detection window. In other words, even if a device listens to all frequencies ie it just uses the voltage output from a knock sensor, it could still detect knock if it is bad enough.
Why Use a Frequency Window?
Because a lot of the time an engine can be knocking and the knocking sound/frequency, may not be the loudest sound coming from the engine. This is especially true in cases of light knock and in cases where engine load and RPM is high.
A good example is this. This is a clip (timestamp 4:57) from a Knock Detective video which plays the raw signal from the knock sensor. Compare that to this sound clip, where irrelevant frequencies have been filtered out. I think with the filtering, it would be easier to detect less severe/light knock. And we would also have more of a chance to detect knock in noisy engine conditions eg high RPM and/or high load.
Datalogging
Referring again to the Knock Detective video clip above. Even when data-logging the raw output voltage from the knock sensor, the results seem good. In normal operation the signal is relatively smooth and when we hear the knock the signal gets jagged (most of the time, although I’m not sure if the log is synced with the video 100% correctly).
In a scrolling datalog feed like that in the video, knock is quite easy to see, however if we were looking at the logs in a histogram format, it might be difficult/impossible to tell what is a normal voltage and what is knock (because we can’t see the voltage in context ie what was happening immediately before and after the noise/voltage). In a histogram view I think it would definitely be preferable to only feed a voltage to the datalogger when knock is occurring, that way we know any voltage we see is knock and not normal engine noise.
Generally – Is Frequency Filtering Better?
I would say yes.
Having said that I think the Knock Detective (device without adjustable frequency window) will probably be able to detect meaningful knock most if not all of the time.
Knock Sensor Types
I’d say before around the year 2000, most factory knock sensors were the resonant type, they were tuned to the specific knock frequency of the engine and in this case, something like Knock Detective could probably perform as well as anything.
Newer knock sensors are the wideband type but even then I would say these wideband knock sensors are designed to focus on common knock frequencies (I presume) so again, I think something like Knock Detective would probably give good results.
In Short
To an extent, frequencies are an over-complication in a knock detection system, especially given knock sensor design.
However, if someone needs more nuanced knock detection, for example they want to see how the knock frequency changes in volume with different loads and timing, frequency detection would be essential.
And if we are analysing datalogs using histograms, again, I would say frequency detection in conjunction with a knock severity indicated via a known voltage level (not simply knock sensor voltage level) is also essential.
But for someone that doesn’t need either of those two things, monitoring all frequencies or a broad spectrum of frequencies would probably be fine.
MoviChip is not in any way connected with FWLR and MoviChip did not receive any incentives from FWLR to write this article.
